To prevent jamming emissions outside a radio frequency band assigned to a mobile communication system, conventional wireless transmission devices generally pass output signals from a final amplifier through a bandpass filter (BPF) to reduce spurious and unwanted emissions outside a management band to acceptable levels.
FIG. 7 is a block diagram showing an exemplary structure of a signal generation device applied in a conventional wireless transmission device.
In a conventional signal generation device 900, an intermediate frequency (IF) circuit 920 firstly generates a modulation signal for an intermediate frequency band that is modulated using a source signal supplied from signal source 910. IF circuit 920 then generates an IF signal from which spurious and unwanted emissions have been suppressed by passing the modulation signal through a surface acoustic wave (SAW) filter having steep out-of-band frequency/attenuation characteristics (hereinafter “attenuation slope”), and outputs the IF signal to a radio frequency (RF) circuit 930.
RF circuit 930 frequency converts the IF signal to a signal for a transmission frequency band, and generates an RF signal by amplifying this signal to a transmission level.
A BPF 940, which is used to eliminate spurious emissions, generates a transmission signal by suppressing spurious and unwanted emissions from the RF signal.
FIG. 8 is a graph showing conceptually the frequency/output level characteristics of output signals from the various components in signal generation unit 900. Signal level and frequency are marked respectively on the vertical and horizontal axes. The curves show the output signals of components in FIG. 7 marked by the same reference signs. The transmission frequency band and intermediate frequency band are normalized on the same relative frequency axis.
Curve 920 shows the output level characteristics of IF circuit 920. The noise level of output signals from IF circuit 920 is steeply attenuated outside the intermediate frequency band as a result of being passed through the SAW filter.
Curve 930 shows the output level characteristics of RF circuit 930. RF circuit 930 amplifies the IF signal sufficiently to surpass a transmission signal level PAll [dBm], although out-of-band (OOB) noise included in the IF signal is at the same time also amplified above an upper tolerance level Plim [dBm/Hz]. Moreover, spurious and unwanted emissions that surpass the amplified OOB noise occur as a result of local spurious signals, intermodulation distortion, and the like.
Curve 940 shows the output level characteristics of BPF 940. The filter used in BPF 940 has a gentler attenuation slope than the SAW filter. Essentially, it is desirable to use a BPF having as steep an attenuation slope as possible in order to reduce the effective range of OOB noise.
However, because in-band attenuation increases as the gradient of the BPF attenuation slope becomes steeper, extremely large outputs are required of RF circuit 930 when using a BPF having too steep an attenuation slope. Also, since the size of BPFs increase proportionate to the steepness of the attenuation slope, a BPF having a reasonably gentle attenuation slope is used in order to achieve device miniaturization.
Noise is attenuated to levels at or below Plim [dBm/Hz] outside of the management band, which includes the guard bands on either side of the transmission frequency band, effectively reducing the output of BPF 940 to PAll [dBm].
Guard bands are conventionally applied as frequency bands not assigned to any mobile communication systems, in order to prevent interference between systems using adjacent frequency bands.
However, a problem with wireless transmission devices using conventional signal generation devices is that they cannot prevent interference in the case of mobile communication systems requiring narrow guard bands.
In particular, securing sufficiently wide guard bands between assigned frequency bands with current systems operating over numerous frequency bands has become difficult due to the tightening of frequency band resources, and has in fact lead to “down” signals transmitted from Personal Handyphone System (PHS) base stations interfering with International Mobile Telecommunications (IMT) 2000 systems employing a Code Division Multiple Access (CDMA) format.
Also, because BPFs in wireless transmission devices using the above conventional signal generation devices also attenuate signals for the transmission frequency band, expensive amplifier components are needed to produce large outputs that compensate for the attenuated amount, thus thwarting device miniaturization and cost-cutting efforts. Moreover, boosting the output of amplifier components requires increased power consumption, which raises electricity and other device operating costs.
With wireless transmission devices used in wireless base stations, in particular, mechanical BPFs are employed to place band restrictions on transmission signals requiring large amounts of power. These mechanical BPFs, being large, heavy and expensive, also stand in the way of device miniaturization and cost-cutting efforts.
Even with mobile telephones and other wireless transmission devices having low transmission signal levels, signals outputted from the final amplifier are generally passed through a BPF to reduce OOB noise before being transmitted.
As such, even if compact, lightweight BPFs with steep frequency band attenuation characteristics are used, expensive amplifier components having large outputs are still needed to compensate for the attenuated amount of in-band signals, thus preventing device cost reductions. Moreover, user convenience is adversely affected by the shortening of battery life resulting from the increased power consumption.